1. Field of the Invention
This invention relates to a heating power supply apparatus for polycrystalline semiconductor rods used for manufacturing semiconductors, and more particularly, to an economical heating power supply apparatus which heats a plurality of polycrystalline semiconductor rods while simultaneously changing the applied voltage in succession and in a wide range, thereby obtaining a stable and balanced current.
2. Description of the Prior Art
For example, in a case of manufacturing silicon of high purity, usually in an apparatus having a base plate covered with a quartz bell-jar, a plurality of not-precipitated silicon carrier rods of high purity are fixed to graphite electrodes, and on the carrier rods a gaseous mixture of hydrogen and trichlorosilane is subjected to thermal cracking so as to precipitate silicon. Since the rod-like carriers have negative resistance particular to the semiconductor material, when they are connected in parallel and heated, a current gathers at the carrier rod of large degree of precipitation and finally only one carrier rod is heated, the others not heated. Therefore, a plurality of carrier rods usually are heated in series connection. Then, a heating power supply apparatus having high voltage necessary for the initial heating and a large current necessary for heating is required when a plurality of carriers are connected in series. However, if one power source is intended to be used for the above purpose, the requisite capacity becomes larger and remarkably lowers the economy, so that a plurality of power sources in combination usually are used. However, the heating power supply apparatus satisfying the above requirement, in fact, has not been accomplished.
For example, a heating power supply apparatus shown in FIG. 1 has a main power source (Tm) having large current capacity and capable of direct-parallel switching and an auxiliary power source (Ts) of high voltage capacity, connected in series, so that when semiconductor rods (L1) through (Ln) are subjected to initial heating, only the main power source (Tm) is used in series connection mode at the intermediate period for heating, and the main power source (Tm) only is used in parallel connection mode at the latter period for heating. However, the FIG. 1 power supply apparatus is still larger in capacity then desired so as to be expensive to produce and to be larger in the required space. Furthermore, the power supply apparatus takes time for power supply switching, such that a current is cut off temporarily during switching thereby to cause abrupt change of the electromagnetic force or temperature change, whereby there is the danger of causing a qualitative defect in the semiconductor rod to be produced.
Another apparatus, for example, switching the power supply by wye-delta connection changeover (Japanese Patent Laid-Open No. Sho 54-80284) is not so different in the effect from that in FIG. 1.